Photodegradation of phenol and salicylic acid by coated rutile-based pigments: a new approach for the assessment of sunscreen treatment efficiency.

The treatments used in many commercial sunscreen pigments (organic additives and inorganic surface coating) to lower the pigments' ability to degrade molecules under photocatalytic conditions are effective in inhibiting the degradation of phenol, but much less effective towards the degradation of salicylic acid. The reason is in the different degradation pathways that phenol and salicylic acid follow under photocatalytic conditions. The treatments are thus effective in inhibiting the degradation pathways initiated by Ti(IV)-*OH(surf) (also named *OH(ads)), as in the case of phenol, but much less effective towards electron-transfer processes involving surface complexes, as in the case of salicylic acid. These results indicate that the techniques currently adopted by the cosmetic industry to develop organic additives for use as pigment treatments are likely to be inadequate. The problem most likely is that treatments are optimized following the inhibition of the photocatalytic degradation of a single model molecule, which results in blocking just one photocatalytic degradation pathway (usually the one initiated by Ti(IV)-*OH(surf)) and not the other (electron transfer processes). The possible implications for sunscreen pigments are briefly discussed, and a new approach for the evaluation of the photocatalytic activity of inorganic sunscreens is proposed.